Printed circuit boards and the like are oftentimes provided with an arrangement that allows another circuit board or the like to be electrically connected thereto. For example, a motherboard or back plane board can be outfitted to allow a daughterboard or plug-in board to be electrically connected thereto. FIG. 1 illustrates a known type of arrangement for electrically interconnecting one printed circuit board like member with another printed circuit board like member.
As seen in FIG. 1, an existing printed circuit board or printed wiring board 10 (e.g., a motherboard or back plane board) has a connector element 12 mounted thereon. The connector element 12 includes a mounting member 14 which can be secured to a mounting block 16 for purposes of being secured to the upper surface of the printed circuit board 10. The mounting member 14 is defined by spaced apart spring-like portions 18 which define therebetween a longitudinal opening 20.
The connector element 12 also includes flexible circuitry 22 disposed on the outer surface of the mounting member 14. The flexible circuitry 22 includes a plurality of generally parallel spaced apart electrical conductors 24. As seen more clearly in FIG. 2, each of the electrical conductors 24 terminates in a contact 26.
The flexible circuitry 22 is positioned on the mounting member 14 in the manner illustrated in FIG. 3 with the ends or leads 28 of the flexible circuitry 22 electrically connected to electrical conductors or contacts 30 on the printed circuit board 10. In that way, an electrical interconnection is established between the electrical contacts 26 of the flexible circuitry 22 and the contacts 30 on the printed circuit board 10. As seen in FIG. 3, an adhesive layer 32 is interposed between the mounting member 14 and the flexible circuitry 22 to secure the flexible circuitry 22 to the mounting member 14.
The longitudinal opening 20 in the connector element 12 is adapted to receive another printed circuit board or printed wiring board 34 (e.g., a daughterboard or plug-in board) which has electrical conductors or contacts 36 disposed thereon. Upon insertion of the printed circuit board 34 into the longitudinal opening 20 in the connector element 12, the electrical conductors or contacts 36 on the printed circuit board 34 mate with and establish an electrical connection with respect to the electrical contacts 26 on the flexible circuitry 22. Thus, an electrical connection is established between the electrical conductors or contacts 36 on the daughterboard or plug-in board 34 and the electrical conductors or contacts 30 on the motherboard or back plane board 10.
In order effect proper electrical continuity between the two printed wiring boards 10, 34, it is necessary that each contact pad 26 of the flexible circuitry 22 make positive physical contact with the electrical conductors or contact pads 36 on the daughterboard or plug-in board 34 that is received in the longitudinal opening 20 of the connector element 12. It has been found, however, that the thickness of the printed circuit board 34 is not always constant from one area of the board to the next. In addition, variations oftentimes exist with respect to the height of adjacent contacts 36 on the printed circuit board 34. When the variations in printed circuit board thickness and contact height are significant, positive physical contact between the electrical conductors or contacts 36 on the printed circuit board 34 and the contacts 26 on the flexible circuitry 22 may be inhibited if the contacts 26 on the flexible circuitry 22 are unable to move relative to one another to compensate for such variations.
In the context of the type of connector element illustrated in FIGS. 1 and 2, it was thought that the adhesive layer 32 which is disposed between the flexible circuitry 22 and the mounting member 14 for purposes of securing the flexible circuitry 22 to the mounting member 14 could also serve the function of allowing the contacts 26 on the flexible circuitry 22 to move relative to one another to compensate for the aforementioned variations in contact height and board thickness. However, it has been found that the adhesive layer 32 is not capable of allowing the individual contacts 26 on the flexible circuitry 22 to move to the extent necessary to compensate for such variations.
One reason for the inability of the adhesive layer 32 to serve the function of a compliant substrate for allowing the individual contacts 26 to move relative to one another is the fact that the adhesive layer is too thin. The adhesive layer 32 applied between the flexible circuitry 22 and the mounting member 14 is on the order of 1-2 mils which has been found to be insufficient to produce the required individualized movement of the contacts 26 relative to one another.
The adhesive layer's inability to serve as a sufficiently compliant substrate also derives from the physical properties of the adhesive layer 32. That is, the adhesive used to secure the flexible circuitry 22 to the mounting member 12 possesses an unacceptably large degree of hardness (i.e., the Shore durometer hardness is relatively high). Thus, the material is not well suited for being readily compressed to allow the individual contacts 26 to move in order to accommodate changes in contact height and wiring board thickness.
To the extent the adhesive material is able to be compressed to a small degree upon the application of a force, it has been found that the adhesive possesses undesirable compression set properties. Generally speaking, compression set refers to a material's inability to be restored to its original form once a compressive force which has been applied to the material is removed. When the daughterboard or plug-in board 34 is inserted into the longitudinal opening 20 in the connector element 12, the wiring board 34 applies a force to the adhesive layer 32. To the extent the adhesive layer 32 is compressed to a relatively small degree upon the application of such a force, the nature of the adhesive is such that the adhesive layer 32 tends to retain its compressed state to a significant extent even when the daughterboard or plug-in board 34 is removed from the connector element 12.
This compression set problem is particularly significant when the daughterboard or plug-in board 34 is retained in the connector element 12 for a long period of time and is then removed and replaced with a different daughterboard or plug-in board. To the extent irregularities in wiring board thickness and/or contact height exist with respect to the first daughterboard or plug-in board 34, the adhesive layer 32 is compressed in a manner corresponding to those irregularities. Then, upon removal of the first daughterboard or plug-in board, it is unlikely that the next wiring board will possess the same irregularities. Consequently, upon insertion of the replacement circuit board, the compression set properties of the adhesive causes the contacts 26 on the flexible circuitry to be incapable of making proper electrical contact with the electrical conductors or contacts on the new wiring board.
In view of the foregoing, it would be desirable to provide a connector assembly for electrically interconnecting two printed circuit board like members which allows individualized compliance of the contacts on the connector assembly to compensate for variations in circuit board thickness and contact height in order to ensure effective electrical contact between the contacts on the connector assembly and the contacts on the printed circuit board like member. It would also be desirable to provide a connector assembly which is not as susceptible to compression set and which can effect the necessary electrical continuity regardless of how long a wiring board is retained within the connector assembly.